The Organic Light Emitting Display (OLED) possesses many outstanding properties of self-illumination, low driving voltage, high luminescence efficiency, short response time, high clarity and contrast, near 180° view angle, wide range of working temperature, applicability of flexible display and large scale full color display. The OLED is considered as the most potential display device.
The OLED display device generally comprises a substrate, an anode located on the substrate, a Hole Injection Layer located on the anode, a Hole Transporting Layer located on the Hole Injection Layer, an emitting layer located on the Hole Transporting Layer, an Electron Transport Layer located on the emitting layer, an Electron Injection Layer located on the Electron Transport Layer and a Cathode located on the Electron Injection Layer. The principle of the OLED is that the illumination generates due to the carrier injection and recombination under the electric field driving of the semiconductor material and the organic semiconductor illuminating material. Specifically, the (Indium Tin Oxide) ITO pixel electrode and the metal electrode are respectively employed as the anode and the cathode of the Display. Under certain voltage driving, the Electron and the Hole are respectively injected into the Electron and Hole Transporting Layers from the cathode and the anode. The Electron and the Hole respectively migrate from the Electron and Hole Transporting Layers to the Emitting layer and bump into each other in the Emitting layer to form an exciton to excite the emitting molecule. The latter can illuminate after the radiative relaxation.
The OLED can be categorized into two major types according to the driving methods, which are the Passive Matrix OLED (PMOLED) and the Active Matrix OLED (AMOLED), i.e. two types of the direct addressing and the Thin Film Transistor (TFT) matrix addressing. The AMOLED comprises pixels arranged in array and belongs to active display type, which has high lighting efficiency and is generally utilized for the large scale display devices of high resolution.
The AMOLED is a current driving element. When the electrical current flows through the organic light emitting diode, the organic light emitting diode emits light, and the brightness is determined according to the current flowing through the organic light emitting diode itself. Most of the present Integrated Circuits (IC) only transmit voltage signals. Therefore, the AMOLED pixel driving circuit needs to accomplish the task of converting the voltage signals into the current signals. The traditional AMOLED pixel driving circuit generally is 2T1C, which is a structure comprising two thin film transistors and one capacitor to convert the voltage into the current.
Generally, the AMOLED pixel driving circuit comprises a drive thin film transistor of driving the organic light emitting diode to emit light. During use, because the aging of the organic light emitting diode and the threshold voltage shift of the drive thin film transistor will lead to the display quality descending of the OLED display device, the prior art compensates the threshold voltage of the drive thin film transistor during use of the OLED display device. The formula for the current flowing through the organic light emitting diode is:
            I      ds        =                  1        2            ⁢              μ        n            ⁢              C        ox            ⁢              W        L            ⁢                        (                                    V              gs                        -                          V              th                                )                2              ,wherein Ids is the current flowing through the organic light emitting diode, and μn is the carrier mobility of the drive thin film transistor, and Cox is a capacitance of a gate oxide layer per unit area, and
  W  Lis a channel width to length ratio, and Vgs is a gate-source voltage of the drive thin film transistor, and Vth is the threshold voltage of the drive thin film transistor; a value of
      1    2    ⁢      μ    n    ⁢      C    ox    ⁢      W    L  is so called a K value of the drive thin film transistor, and the K value also will drift during use of the drive thin film transistor, and the drift of the K value will influence the performance of the drive thin film transistor to lead to the display quality descending of the OLED display device. Therefore, except the compensation to the threshold voltage during use of the OLED display device, it is further required to detect and compensate the K value of the drive thin film transistor to ensure the display quality of the OLED display device during use.